1. Field of the Invention
The present invention relates to a device and a method for determining the firmness of one or more objects, and more particularly to a device and method for non-destructively measuring the firmness of objects, and most particularly to a device and means for non-destructively measuring the firmness of objects using an application of force, with subsequent measurement (or inference) of (i) the volumetric change induced by the force and (ii) the weight (and/or density) of an object(s)(s) so as to determine the firmness of the object(s).
2. Description of Prior Art
As used herein, "firmness" is defined as resistance to deformation, and is a key factor in determining acceptability of a number of products, specifically the quality of food products such as fruits and vegetables. Consumers consider firmness as a predictor of the storing ability and eating quality of fresh fruits and vegetables. Food buyers use firmness when selecting which lot to purchase. Firmness is also a key factor for growers in deciding on harvest dates and in sorting products at packing houses. As used herein, "hardness" may also be used interchangeably with firmness.
One of the primary functions of a fresh fruit and vegetable packing house is to convert a highly variable incoming flow of a product into packages containing products with uniform quality. Many products continue to ripen after harvest, therefore the items packed must be firmer than desired by the end user. Thus, a critical operation of packing houses is to remove riper items which are often the highest quality and more valuable for regional markets but would become soft and cause wholesale buyers to reject the entire shipment when delivered to distant markets. In the apple industry, fruit firmness has long been recognized as an important factor in evaluating apple quality, Finney, Essex, Jr., "Dynamic Elastic Properties and Sensory Quality of Apple Fruit", Journal of Texture Studies 2 62-74 (1971).
Unacceptable variations in firmness frequently occur during food production, manufacturing processes, and product storage. A common method for minimizing variability and for marketing products with uniform firmness is to separate items into groups with similar firmness levels.
Presently, manual separation is the only practical method available to packing houses for firmness sorting. The sorting task is labor intensive, monotonous and inaccurate. Consequently, there is a strong need for development of a mechanical device to measure the firmness of objects during the packing process which will allow separation of objects based on that firmness.
There are several methods of measuring firmness. Generally, such methods measure firmness by destroying the item under test. With these methods, one randomly selects samples from a lot and measures them under the assumption that they represent the total population. The traditional measure of fruit firmness is to measure the force required to penetrate the flesh of the fruit with a penetrometer of standard configuration and tip size. This device destructively measures the firmness of an object by determining the maximum force necessary to penetrate the object with a probe to a predetermined depth. A recent discussion of various destructive test methods is found in a paper by Abbot et. al. (Abbot, J. A., Watada, A. E., Massie D. R., 1976. Effi-gi, Magness-Taylor, and Instron Fruit Pressure Testing Devices for Apples, Peaches, and Nectarines. J. Amer. Soc. Hort. Sci., 101(6):698).
A major disadvantage of destructive testing is that to achieve higher levels of reliability one must destroy greater numbers of the product, and one cannot measure the firmness of every item going through a packing line with destructive test procedures. Furthermore, the ability for a limited sample size (typically less than 0.5% of the total number of individual fruits) to predict the condition of the lot is further weakened because weather and other variables prevent the control of processes (temperature, water stress, nutrient status, etc.) which affect the variability of firmness from fruit to fruit. Thus a lot will have variations that are only partially reduced by mechanical or manual sorting based on size, variety and color. As the fruit ages the variability in the firmness of individual fruits increases and thus the ability to predict the condition of the lot degrades. This is particularly important in determining the time at which controlled atmosphere (CA) storage rooms are opened and once opened whether the fruit (e.g. apples) can be sold on the open market as fresh fruit or has to be diverted to processing. The economic impacts of those decisions are significant for the individual grower and the industry as a whole.
Destructive tests for firmness continue, largely because suitable sensors are not available for measuring the firmness of all items in a lot. Consequently, effort has been expended on several approaches for finding a non-destructive firmness testing method. Such methods have either required mechanical contact between the product and a solid probe or measurement of a secondary property which is subsequently correlated to firmness.
A discussion of the various approaches that have been pursued is provided in U.S. Pat. No. 5,372,030 (Dec. 13, 1994) by Prussia, Astleford and others from the University of Georgia and in U.S. Pat. No. 4,061,020 (Dec. 6, 1977) by Fridley, Chen and others from U.C. Berkley. The methodology in the two referenced patents are basically point compression methods which relate the compression of a point or a spot on the surface of the test object (fruit) to the firmness of the entire fruit. A common defect of the point compression technique and the instruments that would be based on this technique, is the fact that only a single point or at best a few points are measured. The point deformation methods typically use a hard ball or pin to deform the fruit at the measurement point.
Alternatively, the technique in U.S. Pat. No. 5,372,030 uses air to deform the fruit at the measurement point and a laser beam to measure the deflection, but it is still basically a point deflection method. It is well documented that the apple is not homogeneous in regard to firmness test results obtained from the destructive punch test firmness determination and would not be expected to be uniform with regard to point compression as a measure of firmness. Most researchers (Abbott, 1976, 1994) take multiple measurements on the apple in order to average out known variations. In the experiments conducted in support of the development of this patent application, it has been noted that with four destructive punch tests, the relative standard deviation of the four tests has been on the order of eight to ten percent (a variability of approximately. 1 to 2 lb. of compression per apple) although some apples exhibit up to 25% difference (&gt;3 lb.) from one location on the apple to the other. This data is consistent with that of other researchers (Abbot, Judith A., "Firmness Measurement of Freshly Harvested `Delicious` Apples by Sensory Methods, Sonic Transmission, Magness-Taylor, and Compression", J. Amer. Soc. Hort. Sci. 119(3):510-515, 1994) and illustrates that there is significant point to point deviation around the apple in the underlying property (firmness) that is being measured. The property that the consumer and the industry should be most interested in is the general or average firmness of the entire apple. However, other methods that are currently proposed as non destructive firmness measuring techniques typically make point measurements and extrapolate the results from that point measurement to the whole apple.
There is need for a method and apparatus that can test the individual object (e.g. apple) for the desired property of firmness. There is a further need to be able to test non-destructively, multiple objects as a group to obtain the average property of firmness without testing each individual object (e.g. apple). A variant of the apparatus may be used for testing individual objects on a production or processing line, for example apples on the packing line.